Insiders have described how the Prime Minister's ex aide Lee Cain took charge after some of the Government's highest office holders fell ill with Covid-19.
Downing Street advisers said Covid turned No 10 into a 'plague pit' where almost all the staff caught the infection.
According to the Guardian, the former director of communications Lee Cain was effectively 'running the country' for 'quite some time' when the infection spread through No 10.
Sources told the newspaper that for a number of days, aides turned to Lee Cain for direction.
Sources say the former director of communications Lee Cain (pictured, right, with Dominic Cummings) was 'running the country' when Government's highest office holders had Covid-19
During his time at The Mirror Lee Cain dressed up as a chicken to harass politicians including Conservative leader David Cameron (pictured) during the 2010 general election campaigns
One said: 'No 10 was a plague pit. No one outside the postcode quite knows how bad it got in there.'
While another added: 'Lee was running the country, genuinely, for quite some time.'
Prior to his role in Government, Mr Cain worked for the Mirror where he dressed up as a chicken to harass former Tory leader David Cameron during the 2010 general election campaign.
The Guardian reports that almost all the staff at Downing Street became infected at one time or another.
One adviser said : 'You'd got to a meeting expecting to see someone important and they were gone – and you knew why.
'It started off more surreal than scary, but it became very scary once the PM was so ill.'
Reports suggest there was little social distancing at No 10 in the build up to lockdown and staffers admitted it took time for the pandemic to register on the Government's radar.
Lindsay Hoyle, the newly elected Speaker of the House of Commons, told the Guardian: 'I don't think we got a picture [for a while] for how big of scale this was, or how it was going to affect the house.
'Like everybody else, we were underestimating how bad it was going to be. I'll be very open, very honest about that. It was lack of understanding of what we were facing.'
Despite increasing concern about the virus, Government officials continued to hold meetings in close contact even after several members had tested positive.
In March, Boris Johnson held a meeting with staff, including Cummings and Prof Whitty, as well as Labour leader Jeremy Corbyn to discuss the coronavirus bill - all crowded round one desk.
Lee Cain (pictured, right, with Prime Minister Boris Johnson) announced he would resign in November last year after infighting at No 10 just hours after he was touted for a promotion
Later that month, Boris Johnson and Matt Hancock both tested positive for coronavirus as well as a number of staffers who all tried to continue working, according to the Guardian.
Longstanding Johnson aide Lee Cain, a Dominic Cummings loyalist, announced he was resigning in November despite being touted for promotion to No10 chief of staff just hours earlier.
The departure was the culmination of a bitter power struggle inside Mr Johnson's top team, with rival factions battling for supremacy even as the government struggled to tackle the coronavirus crisis.
Mr Cummings had pushed for his ally to be appointed despite warnings from the PM's fiancée Ms Symonds - herself a former Conservative Party head of media - that it would be 'a mistake' given how the campaign against the pandemic had gone so far.
She is said to have complained the No10 operation was being run in an 'uncollegiate' way and the PM was not getting 'good advice'.
There are also claims that Allegra Stratton, Downing Street's new on-screen press secretary, and senior aide Munira Mirza were against the move. It would have meant the PM's core circle being exclusively male.
Ms Stratton was said to have only agreed to take the job if she reported directly to the PM, not Mr Cain - leaving him feeling 'wholly undermined'.
Mr Cain was said to have opposed her appointment and the pair had not spoken since she arrived in the role.